“The manure the Chinese use to feed fish is frequently contaminated with microbes like salmonella,” says Doyle, who has studied foodborne diseases in China.

On a sweltering, overcast day in August, the smell of excrement is overpowering. After seeing dead fish on the surface, Chen, 45, wades barefoot into his murky pond to open a pipe that adds fresh water from a nearby canal. Exporters buy his fish to sell to U.S. companies.

Yang Shuiquan, chairman of a government-sponsored tilapia aquaculture association in Lianjiang, 200 kilometers from Yangjiang, says he discourages using feces as food because it contaminates water and makes fish more susceptible to diseases. He says a growing number of Guangdong farmers adopt that practice anyway because of fierce competition.

“Many farmers have switched to feces and have stopped using commercial feed,” he says.

5. The difference: Dirt and roots are better filters

And chemically they help in breaking down pathogens. Cow feces is allowed to "mature" before being put on your plants, otherwise the high nitrogen content would "burn" the plants. The "maturing" often includes temperature extremes that help kill pathogens.

Pig feces is particularly dangerous because of how easily porcine diseases are spread to humans.

Unlike the plants, which have the various buffers discussed above, fish and shrimp are ingesting the pathogens and becoming infected. Then they are shipped directly to your local WalMart, where low, low prices are everyday!